$re = '/(?<!http.*)\/(.*?)\//m';
$str = 'Lorem // ipsum
https://www.google.com/search?q=lorem+ipsum
/lorem:ipsum/
consecteur /adipiscing/ eu/turpis//velit/ ut/rhoncus/
tincidunt: eget; nibh nam: facilisis; ligula: lacinia
1.
sed at pulvinar ante.
"cras nec nibh lacus"
suspendisse potenti
ut pharetra urna sed. "proin convalis uris"
phasellus tincidunt
mauris justo iaculis. "vivamus scelerisque"
';
$subst = "[$1]";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php